The composition of natural gas delivered through the pipeline varies with time and location around the USA. These variations are known to affect engine performance and emissions through changes in fuel metering characteristics and knock resistance of the fuel. High output, low emissions natural gas engines are being developed that take advantage of the high knock resistance of natural gas. These optimized engines are operated close to knock-limited power where changes in fuel knock resistance can cause operational problems. Octane tests were conducted on natural gas blend fuels using a CFR octane rating engine.Two relationships between motor octane number and fuel composition were established. A correlation for motor octane number versus the reactive hydrogen-carbon ratio was developed, and octane weighting factors, which used the molar composition of the fuel to predict motor octane number, were also found. In addition, a correlation for motor octane number versus methane number was also developed. These correlations provide practical methods for determining octane number and significant insight into the effects of heavy hydrocarbons on octane number of the fuel. Further research is needed to determine the impact of more complex fuel blends on octane number.